The present invention relates to plants for the production of blown films, i.e. plastic films extruded in bubble shape, and in particular to a device and a relevant method for adjusting the thickness profile of the film. Prior art devices that provide said adjustment operate essentially in two ways, namely through heating elements that increase the temperature of the molten plastic material close to the die lip or of the flow of cooling fluid, typically air, or through actuators that adjust the flow rate of the flow.
In the first type of device the heating elements can be arranged in the extrusion head die and/or in the cooling ring, while in the second type of device the actuators can be arranged in the cooling ring or in a dedicated adjustment ring connected to the cooling ring or independent therefrom (typically interposed between the extrusion head and the cooling ring). Each known arrangement has drawbacks that limit its effectiveness and range of application.
When the heating elements are arranged in the extrusion head die, in a discrete number evenly distributed along the perimeter, a first drawback stems from the temperature increase in the die lip that can result in a degradation of the plastic material due to exposure to an excessive temperature for a significant time, consequently altering the optical and mechanical properties of the film. Moreover, the deposition of degraded material on the sliding surface of the die requires frequent downtimes to perform the cleaning of the extrusion lip with costly inefficiencies and rejects.
A second drawback consists in the fact that the device operates in a satisfying way in the case of angularly extended and distributed corrections but does not lend itself to point corrections of small angular extension since the metal of the die is thermally conductive and this prevents from obtaining significant temperature gradients in adjacent regions of the die. Furthermore, the adjustment is slow due to the high thermal inertia that opposes the temperature change of a body of solid metal.
A further limit is given by the unidirectional adjustment of the thickness towards smaller values since only a positive variation of the temperature with respect to a reference temperature is available, whereas a symmetrical system of die cooling that produces a negative variation of the temperature is not available.
It is possible to obtain an adjustment device that is quicker, more precise and effective by arranging the heating elements in the cooling ring since the temperature change of the heating elements and of possible radiating bodies connected thereto is quicker in that the masses of the bodies to be heated are smaller than the die mass. Moreover, by arranging the heating elements in the ring cooling flow it is possible to achieve a cooling of the heating elements in a comparatively quicker way with respect to similar heating elements arranged in the metal of a die.
Therefore it is a common practice to set a film reference temperature (so-called “set point”) by keeping all the heating elements turned on at a fraction of their maximum power, indicatively 15-25%, in order to allow for temperature corrections both positive, through an increased heating of the air flowing in the ring, and negative through a decreased heating of the air thus resulting in an increased film cooling. In this way it will be possible to perform corrections of the circumferential profile of the film either by increasing its thickness (lower heating) in the case of regions below the average value, or decreasing it (higher heating) where a thickness higher than the average value is detected.
Such a device allows therefore a bidirectional adjustment of the thickness but has the serious drawback of implying a significant energy waste and a low efficiency. In fact, the air flowing in the ring that is used to cool the bubble usually comes from suitable cooling plants that lower its temperature so as to increase the cooling effectiveness to maximize the hourly output of the line, although it might also be cooled directly in the ring as shown in JP 05104623. Imposing a base heating, in order to allow the bidirectional thickness adjustment, to the previously cooled air leads therefore to a high power consumption both for the cooling and the subsequent partial heating of the air, which results in a poor energy efficiency.
It should also be considered that the heating elements in the cooling ring are present in a number greater than those that can be arranged in the extrusion die, for the same die diameter, and this allows to control a greater number of adjustment points along the perimeter. By way of example, considering a typical device with a hundred heating elements having a maximum power of 200 W each that are set at 20% to obtain the reference temperature, the basic hourly consumption of the device is equal to 4 kWh and the maximum theoretical hourly consumption is equal to 20 kWh.
In the second type of device the thickness adjustment is based on the change in the flow rate of the cooling air, said change being also possibly used to adjust the Venturi effect that keeps the film close to the cooling ring as shown in EP 0914928. Through motorized shutters or valves it is possible to modify the thermal cooling exchange between air and film. The advantages of this type of device are a higher adjustment velocity, the bidirectional adjustment and a negligible consumption due only to the activation of the actuators. On the other hand, the size of the actuators results in general in the possibility of installing only a lower number of adjustment points with respect to the above-described arrangement of the heating elements in the cooling ring.
Another drawback of this solution is the risk of compromising the stability of the bubble when the flow rate of the cooling flow is greatly increased/decreased in order to increase/decrease the film thickness up to the ends of the possible adjustment range. In fact, the air flowing on the inside and on the outside of the bubble forming cone not only affects the cooling capacity of the device but is also responsible for the pressure conditions on the ring inserts that determine the constancy of the fluid passage and the attractive force between the parts.
Changing the air flow rate implies changing also the flow thickness and the attractive force of the bubble on the forming cone, whereby when increasing the distance between the ring and the bubble the latter can be less guided and less stable thus jeopardizing the effectiveness of the adjustment itself and sometimes the stability of the entire process. On the other hand, an excessive decrease in the flow thickness implies the risk of contacts between the film and the forming cone with resulting surface alterations of the film characteristics and possible breaking of the bubble itself.
As a consequence, if the reference flow is set at a flow rate of the cooling air such that at the minimum flow rate there is no adhesion of the film to the forming cone, there is the risk that bubble instability occurs at the maximum flow rate. Vice versa, if the reference flow is set at a flow rate of the cooling air such that at the maximum flow rate there is no bubble instability, there is the risk that adhesion of the film to the forming cone occurs at the minimum flow rate. On the other hand, limiting the change in the flow rate in order to prevent both the adhesion risk and the instability risk would result in a thickness adjustment range that is too narrow.
In case the actuators are arranged in a dedicated adjustment ring, as mentioned above, the air used for the thickness adjustment may come from a suitable flow generator such as a fan or a compressor, or it may be a partial flow derived from the same circuit of the cooling ring. It should finally be noted that cooling rings usually provide one or more flow splits, with the cooling flow that is typically split into a bottom flow and a top flow respectively flowing on the inside from the bottom edge of the forming cone and on the outside thereof, converging at the cone top edge. It is clear that the above-illustrated principles of thermal or volumetric adjustment remain unchanged regardless of the fact that they are applied to the total flow to a partial flow.
Descriptions of these film thickness adjustment systems can be found in various documents, for example EP 0524697 describes a cooling device for extrusion heads that in a first embodiment is provided with means for the adjustment of the flow rate of the cooling air flow and in a second embodiment is provided with means for the adjustment of the temperature of said flow. EP 1982819 discloses a similar cooling device that in addition to said two embodiments also provides a third embodiment obtained by combining the first two, i.e. with two cooling rings that adjust two distinct air flows according to the thermal and volumetric modes respectively.
WO 2012/080276 discloses a cooling device that is different from the above-mentioned devices in that the thermal and volumetric adjustments are carried out simultaneously and in combination, since the former automatically controls the latter. More specifically, the adjusting element consists of a radial heating element and a bimetallic element that extends thereon and is secured thereto at one end whereas the other end remains free, whereby the bimetallic element acts as a flow adjustment valve by restricting the air passage cross-sectional area when it is heated. In this way, the increase in air temperature also implies a decrease in the flow (and therefore a decrease in the film thickness) and vice versa the decrease in air temperature also implies an increase in the flow (and therefore a increase in the film thickness) since the two adjustment modes are combined in the same direction.
Such an arrangement makes the adjustment system quicker yet also less sensitive and more limited as adjustment range, since the combination of the two thermal and volumetric adjustments does not leave room for further interventions.